1. Field of the Invention
The invention relates to a sensor or actuator structure and a method of manufacturing a sensor or actuator structure with localised electrical insulation in a substrate wafer. The invention relates, in particular, to an accelerometer structure and its method of manufacture. By accelerometer, one understands an acceleration sensor comprising, for the main part, a mobile element of mass m and means of measuring the force F=m.gamma. exerted on the mobile element and due to the acceleration .gamma. of a body in motion on which the accelerometer is mounted.
Today, acceleration is a parameter which has become more and more useful and necessary to know for industrial requirements, notably in the space and aeronautical fields, and also for applications for the public at large such as automobile applications for the control of active suspension or of air bags.
The development of these latter applications necessitates a very large reduction in manufacturing costs while at the same time maintaining acceptable metrological properties of the sensors.
In a general way, the accelerometers of the invention can be used in all fields where one wishes to measure the acceleration of a body in motion.
The invention also relates to structures of sensors such as gyrometers or pressure sensors, as well as actuators.
2. Discussion of the Background
A first known type of accelerometer is described, for example, in FR-A-2 700 065 (thereafter "Document (1)").
Document (1) refers to an accelerometer manufactured on a Silicon On Insulator type substrate. Such a substrate comprises a surface layer of silicon and a support layer, also made of silicon and separated from the surface layer by a layer of silicon oxide called an embedded insulating layer.
The mobile elements of the accelerometer, in the same way as the measurement means are etched directly in the surface layer which is kept integral with the support layer by means of the embedded insulating layer. The embedded insulating layer also provides electrical insulation between the different parts of the accelerometer.
A second known type of accelerometer is described, for example, in FR-A-2 558 263 (therafter "Document (2)").
The accelerometer type conforming to the information disclosed in document (2) is produced by micro-lithography, directly into a massive substrate. The elements of the accelerometer, and in particular its mobile seismic mass, are defined by etching of the substrate using a suitable mask.
The different parts of the substrate are fitted with metal electrodes forming capacitive means of measuring the displacement of the seismic mass in motion. The electrical insulation of the electrodes is provided by the substrate itself which is for example, made of quartz.
The accelerometers described above are of complex construction, either because of the use of a multi-layer substrate of the Silicon On Insulator type, or because of the need to provide conducting electrodes capable of forming the measurement means, on various parts of the accelerometer.
The complexity of the construction constitutes an obstacle to the miniaturisation of the devices, to the provision of high production efficiency and to the cost reduction of the accelerometers obtained.